Low density polyethylene (LDPE) resins for extrusion coating and extrusion lamination on paper, board, aluminum, etc., are designed with broad MWD (molecular weight distribution) and low extractables. In extrusion coating applications, the polymer is processed at high temperature conditions, typically above 260° C. and below 350° C. Broad molecular weight distribution (MWD) resins with a very high molecular weight fraction are used for good processability during coating (neck-in and drawdown balance). Low extractables are needed to reduce undesirable taste and odor issues in the final product. Low extractables are also needed to reduce smoke formation during the processing of the resin, especially during coating processes run at high temperatures.
Typically LDPE resins with broad MWD are made using autoclave reactors or a combination of autoclave and tube reactors. Broad MWD resins can be achieved in autoclave reactors by promoting long chain branching, and through the inherent residence time distribution, by which molecules will undergo shorter (low molecular weight) or longer (high molecular weight) growth paths.
The autoclave and tubular reactor systems differ from each other in respect to residence time distribution, which is typically more uniform for tubular reactors and dispersed for autoclave reactor zones. Polymerization conditions like temperature, pressure and polymer concentrations vary widely in tubular reactor systems, and are uniform or are less differentiated for autoclave reactor systems.
The uniform residence time in tubular reactors leads to narrower MWD, therefore very broad MWD can only be achieved in tubular reactors by applying extremely differentiated polymerization conditions, for example, as described in International Application No. PCT/US12/064284 (filed Nov. 9, 2012), and/or application of a branching/cross-linking agent, for example, as described in U.S. Pat. No. 7,820,776. The use of extreme process conditions and/or branching/cross-linking agents can lead to high melt strength tubular, low density polyethylene suitable for extrusion coating applications. These tubular polyethylenes will have a specific composition (e.g. density), and functionality as determined by the applied process conditions, type and level of branching agent and/or comonomer. Undesirable gels in the polymer can be an issue, resulting from the use of branching or cross-linking agents.
In extrusion coating applications, the following product and application properties are, among others, of importance: coating performance at variable processing speeds, adhesion to the substrate, barrier properties, and seal formation. The coating performance at variable processing speeds will depend mainly on the viscoelastic properties of the polymer, while adhesion, barrier, and sealing properties will also depend, in addition to the viscoelastic properties, on the density and functionality of the polymer.
EP0792318A1, EP1777238A1, EP2123707A1, and EP2123707A1 describe compositions of polymers suitable for use in extrusion coating applications, in which the viscoelastic performance is contributed by an autoclave-based LDPE, as a minor blend component, and in which the overall density and the application performance are determined by the major (non-LDPE) blend component.
The autoclave blend components used in such compositions have melt strength properties exceeding the normal autoclave products used in non-blend extrusion coating applications, and are therefore even more difficult to match in a tubular LDPE process. This favorable viscoelastic performance of these autoclave blend components is achieved by extremely broad, and, in some cases, bimodal MWD. However, the presence of an ultra high molecular weight fraction in these resins has a negative impact on the optical appearance in the final extrusion coating application. Furthermore, an autoclave process typically operates at lower conversion levels, and is more capital/energy intensive than a tubular process.
Thus, there is a need for new ethylene-based polymers with broad MWD, which are suitable as a blend component in compositions to be used in extrusion coating applications (sufficient melt strength), but lacking the ultra high molecular weight fraction of broad MWD autoclave resins, and which can be made in a tubular process.
EP0792318A1 discloses an ethylene polymer composition comprising from about 75 to 95 percent, by weight, of at least one ethylene/a-olefin interpolymer, and about 5 to 25 percent, by weight, of at least one high pressure ethylene polymer. The ethylene/α-olefin interpolymer is selected from the group consisting of a substantially linear ethylene polymer, a homogeneously branched linear ethylene polymer and a heterogeneously branched linear ethylene polymer, wherein the ethylene/α-olefin polymer is characterized as having a density in the range of 0.85 g/cc to 0.940 g/cc. The high pressure ethylene polymer is characterized as having a melt index, I2, less than 6.0 g/10 minutes, a density of at least 0.916 g/cc, a melt strength of at least 9 cN, a Mw/Mn ratio of at least 7.0, and a bimodal molecular weight distribution as determined by gel permeation chromatography. The ethylene polymer extrusion composition has a melt index, I2, of at least 1.0 g/10 minutes.
U.S. Pat. No. 7,776,987 discloses a composition based on low melt index LDPE (typically in the range of 0.2 to 1.0 g/10 min), in the amount of 10 to 25%, together with a high melt index linear polyethylene, where the melt index of the linear polyethylene is in the range of 20 to 100, preferably 30 to 40, and is suitable for use in extrusion coating. The composition comprises polymeric material having certain rheological and gel permeation chromatography (GPC) properties, and exhibits reduced neck-in when used in extrusion coating. This neck-in is independent of melt strength, thereby facilitating improved extrusion processes.
EP1777238A1 claims the use of an autoclave based LDPE with a melt index within the range of 2.5 to 10.0 g/10 minutes, with certain dynamic mechanical spectroscopy rheological properties, as a blend component in compositions suitable for extrusion coating applications. A related patent, EP2123707A1, discloses 2-30 wt. % of the above autoclave LDPE in blends with a tubular LDPE having a melt index of 2 to 8.
US2007/0225445 discloses polymer blends composed of 25 to 75 wt. % homopolymer LDPE, produced in a tubular reactor, and 75 to 25 wt. % of ethylene homopolymer LDPE produced in a high pressure autoclave reactor, and provided that each homopolymer is removed from the reaction zone prior to being blended together. The blends, so formed, have a good combination of neck-in and adhesion properties. When the tubular product is blended with the autoclave product in blend ratios varying from 0.7:0.3 to 0.3:0.7, the neck-in of the polymer web varied between 165 to 95% versus the neck-in of the autoclave reference. The pure (100%) tubular neck-in was 305% times the neck-in of the autoclave reference.
The presentation of J. Bosch (“The Introduction of Tubular LDPE to the Extrusion Coating Market and the Specifics of the Product”) at the 12th TAPPI European Place Conference, in 2009, discloses the differences between autoclave and tubular based resins, and the consequences on extrusion coating performance. Further this reference explains the need to develop non-blend, tubular resins for extrusion coating applications.
Conventional high melt strength ethylene-based polymers used in compositions suitable for extrusion coating are made in the autoclave process with very broad MWD and with the presence of an ultra high molecular weight fraction, which has a negative impact on the optical appearance in the final application. Furthermore, an autoclave process typically operates at lower conversion levels, and is more capital/energy intensive than a tubular process. Conventional tubular products lack melt strength to provide the desired viscoelastic properties to extrusion coating compositions made with these low melt strength resins.
Thus, there is a need for new ethylene-based polymers with high melt strength, which are suitable as a blend component in compositions to be used in extrusion coating applications, but lacking the ultra high molecular weight fraction of broad MWD autoclave resins, and which new polymers can be made in a tubular process. These needs and others have been met by the following invention.
SUMMARY OF INVENTION
The invention provides a composition comprising a first ethylene-based polymer, formed by a high pressure, free-radical polymerization process, and comprising the following properties:
a) a Mw(abs) versus I2 relationship: Mw(abs)<A×[(I2)B], where A=5.00×102 (kg/mole)/(dg/min)B, and B=−0.40; and
b) a MS versus I2 relationship: MS≧C×[(I2)D], where C=13.5 cN/dg/min)D, and D=−0.55.